1. Field of the Invention
The present invention generally relates to a photocoupler apparatus, and more particularly, to a photocoupler apparatus which can shorten the switching time of output contacts.
2. Description of the Related Art
There are photocoupler apparatuses which converts an optical signal from a light-emitting diode driven by an input signal into an electric signal by a photoelectromotive force diode array photocoupled to the light-emitting diode, and drives a metal oxide semiconductor field effect transistor (hereinafter referred to as MOSFET) by this electric signal so as to acquire a contact signal. The conventional photocoupler apparatuses may be constituted as follows.
FIG. 1 illustrates a photocoupler as disclosed in U.S. Pat. No. 4,227,098. A light-emitting diode 14, connected to input terminals 12a and 12b, is photocoupled to a photodiode array 16 serving as a photoelectromotive force diode array. The photodiode array 16 has one end connected to the gate of a gate insulating type output MOSFET 18, and the other end connected to the substrate and source of the MOSFET 18. Output terminals 20a and 20b are respectively connected to the drain of the MOSFET 18 and the substrate and source thereof. Further, a resistor 22 having a resistive impedance is connected in parallel to the photodiode array 16.
When the thus constituted photocoupler is turned on, electric charges are accumulated in an electrostatic capacitor between the gate and substrate of the output MOSFET 18 by the photodiode array 16. When the photocoupler is turned off, the resistor 22 serves to discharge the accumulated charges. Without this resistor 22, the charges which have been accumulated in the electrostatic capacitor between the gate and substrate of the output MOSFET 18 would be discharged through the photodiode array 16. Such discharging would drastically slow down the discharging speed and significantly increase the time T.sub.off from a point when the input current flowing through the light-emitting diode 14 is cut off to a point when the output MOSFET 18 returns to an OFF status.
When a current is permitted to flow through the light-emitting diode 14 to render the output MOSFET 18 to an ON status, the presence of the resistor (resistive impedance) 22 is not preferable to permit bypassing of the current generated by the photodiode array 16. The value of the resistor 22 should be increased in order to shorten the time T.sub.on from a point where an input current is permitted to flow to the light-emitting diode 14 from the input terminals 12a and 12b until the output MOSFET 18 is rendered ON, as well as to reduce the minimum input current or operating current I.sub.FT required to render the output MOSFET 18 ON.
In order to shorten the aforementioned time T.sub.off, however, the value of the resistor 22 needs to be set small, thus presenting a trade-off between T.sub.on (I.sub.FT) and T.sub.off of the output MOSFET 18. The trade-off therefore prevents improvement of the characteristics of both T.sub.on and T.sub.off while shortening the switching time of the contacts of the output MOSFET 18 or realizing a high-speed operation.
A photocoupler shown in FIG. 2 and disclosed in U.S. Pat. No. 4,390,790, Published Unexamined Japanese Patent Application 57-107633 and EP 0048146, is designed to shorten the contact switching time of the output MOSFET 18.
A light-emitting diode 14, connected to input terminals 12a and 12b, is photocoupled to a first photodiode array 24. The photodiode array 24 has one end connected to the gate of a gate insulating type output MOSFET 18, and the other end connected to the substrate and source of the MOSFET 18. Output terminals 20a and 20b are respectively connected to the drain of the output MOSFET 18 and the substrate and source thereof. Both ends of the photodiode array 24 are further connected to the drain and source of a normally-ON drive transistor (FET) 26, respectively. Further, a second photodiode array 28, which has the illustrated polarity and is photocoupled to the light-emitting diode 14, is connected between the gate and source of the drive transistor 26.
According to the thus constituted photocoupler, a capacitance between the gate and substrate of the drive transistor 26 is smaller than that between the gate and substrate of the output MOSFET 18. It means that the necessary light current for the second photodiode array 28 can be smaller than that for the first photodiode array 24. This can reduce the area of each diode in the second photodiode array 28 and can thus enlarge the diode-occupying area in the first photodiode array 24, thus contributing to optimization of the efficient use of the space.
However, the second photodiode array 28 is required only to drive the drive transistor 26 and inevitably increases the number of the necessary photodiode arrays, which results in an increase in the cost of the photocoupler.
A photocoupler as disclosed in Published Unexamined Japanese Patent Application 63-99616 has been developed as a solution to overcome the above shortcoming. As shown in FIG. 3, a light-emitting diode 14, connected to input terminals 12a and 12b, is photocoupled to a photodiode array 16. The photodiode array 16 has one end connected to the gate of a gate insulating type output MOSFET 18, and the other end connected to the substrate and source of the MOSFET 18 through a resistor 30 having a resistive impedance. Output terminals 20a and 20b are respectively connected to the drain of the output MOSFET 18 and the substrate and source thereof. A normally-ON drive transistor (FET) 26 has its drain connected to the gate of the output MOSFET 18 and its source connected between the substrate and source thereof, with its gate connected between the photodiode array 16 and the resistor 30.
According to this photocoupler, the aforementioned problem is overcome by driving the drive transistor 26 with the potential difference across the resistor 30 (resistive impedance), which occurs by a current flowing across the resistor 30. In other words, in this circuit, a voltage applied between the gate and substrate of the output MOSFET 18 is the minimum voltage produced across the resistor to drive the drive transistor 26, subtracted from the voltage across the photodiode array 16. This circuit shown in FIG. 3 differs in the following points from the one shown in FIG. 2 which employs the second photodiode array only to drive transistor 26.
If the total number of the diodes in the photodiode array of the prior art circuit shown in FIG. 3 equals the total number of the diodes in the photodiode arrays in the prior art circuit shown in FIG. 2 (i.e., the sum of the diodes in the first photodiode array 24 and those of the second photodiode array 28), the former circuit (FIG. 3) has a higher voltage applied between the gate and substrate of the output MOSFET 18. It is therefore possible to drive a MOSFET having a high threshold voltage V.sub.th, the gate-substrate voltage necessary to drive the drive transistor 26. In addition, as mentioned earlier, the gate-substrate capacitance of the drive transistor 26 is smaller than that of the output MOSFET 18.
According to the thus constituted photocoupler, however, a single photodiode array replenishes the gate-substrate capacitance of each of the drive transistor 26 and the output MOSFET 18, so that every diode constituting the photodiode array 16 should supply a large light current. In other words, since there are no separate photodiode arrays for respectively driving the output MOSFET 18 and the drive transistor 26, the photodiode array for driving the drive transistor 26 cannot be made smaller. This necessitates that the area of the photodiode array 16 be increased, thus increasing the overall space for the circuit.
In addition, as the resistor (resistive impedance) 30 is used as an impedance element, the resistive impedance limits the charge current to be supplied between the gate and substrate of the output MOSFET 18 when this current is large. This also limits shortening of the aforementioned time T.sub.on.
To prevent such undesirable limitation, the resistor 30 in FIG. 3 may be replaced with a parallel circuit of a resistor and a diode array which does not include photodiodes. In this case, a large current involved to switching on or off a transistor may flow using the diode array as bypassing means. Driving the drive transistor 26 however requires a voltage obtained from a voltage drop for at least three to six diodes. This needs an area for the diode array serving as the bypassing means and further deteriorates the effective use of the total circuit space and the circuit characteristic, thus resulting in an increase in the cost of the circuit.